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Cacat Pada Kristal Logam
1
2
Crystals are like people, it is the defects in them
which tend to make them interesting! - Colin Humphreys.
Cacat kristal (Defects) di dalam logam:
0D, Cacat titik (Point defect)
vacancies
interstitials
impurities, weight and atomic composition
1D, Dislokasi/cacat garis (Linear Defect)
2D, Cacat Bidang (Planar Defect)
3D, Cacat Volume (Volume Defect)
3
Kristal yang sebenarnya (Real crystals) tidaklah
pernah sempurna karena didalam strukturnya selalu
terdapat cacat (defect)
Schematic drawing of a poly-crystal with many defects
by Helmut Fll, University of Kiel, Germany.
Cacat Kristal (Defect)
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Why Study Defects?
Defects memiliki pengaruh yang sangat besar dalam menentukan sifat suatu material
Sebagai contohnya: sifat mekanik dari logam murni sangat berbeda jika dibandingkan dalam
bentuk paduan (alloy). Perunggu (70% Cu- 30%
seng) dengan Cu murni yang bersifat lunak
Contoh lainnya: komponen IC (semikonduktor) didalam perangkat elektronik sangat dipengaruhi
oleh konsentrasi impurity di dalamnya.
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Jenis Defect
Jenis-Jenis Defect Berdasarkan
Dimensinya:
0D, Point defects: Cacat titik yang paling sederhana adalah
kekosongan disebabkan adanya atom yang hilang
dalam kristal (ex: Vacancy, Interstitial, Impurity)
1D, Linear defects: Adanya sisipan satu bidang atom tambahan
didalam struktur kristal akibat tekanan dan
tegangan (ex: edge, line, screw dislocation)
2D, Planar defects: Cacat yang ditandai adanya pemisahan batas 2D
yang memiliki kristal berbeda (ex: grain
boundaries, external surfaces)
3D, Volume defects: extended defects (ex: pori, retak/crack, inklusi)
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Point defects: vacancies &
interstitials
Self-interstitials
Vacancy
Vacancy - kisi yang kosong akibat kehilangan
atom
Interstitial adanya atom yang menempati
celah kosong diantara atom. Defect jenis ini
dapat berupa atom yang sama (self interstitial)
atau interstisial dari atom pengotor
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Jumlah ruang kosong pada kondisi
equilibrium (Vacancy) akibat vibrasi
termal:
Bagaimana Jumlah Vacancy di
hitung?
Ns = jumlah regular lattice
kB = konstanta Boltzmann
Qv = energi yang diperlukan untuk membentuk
ruang kosong pada kisi kristal ideal
T = temperature dalam Kelvin.
At room temperature in Cu: one vacancy per 1015
atoms.
Just below the melting point: one vacancy for every
10,000 atoms.
Tk
QexpNN
B
vsv
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Estimate number of vacancies in Cu at room T
kB = 1.38 10-23 J/atom-K = 8.62 10-5 eV/atom-K
T = 27o C + 273 = 300 K.
kBT = 300 K 8.62 10-5 eV/K = 0.026 eV
Qv = 0.9 eV/atom
Ns = NA/Acu
NA = 6.023 1023 atoms/mol
= 8.4 g/cm3
Acu = 63.5 g/mol
Tk
QexpNN
B
vsv
3
223
23
s cmatoms108
molg
5.63
cmg
4.8mol
atoms10023.6
N
atomeV026.0atomeV9.0
expcm
atoms108N
3
22
v
37 cmvacancies104.7
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Distorsi besar dalam kisi kristal di sekitarnya
Energy pada self-interstitial formation
~ 3 x lebih besar daripada vacancies (Qi ~ 3Qv)
Konsentrasi atom pada self-interstitials sangat
kecil (< 1/ cm3 at 300K)
Self-interstitials
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3
4
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Point defects: self-interstitials, impurities
(1) vacancies
(2) self-interstitial
(3)interstitial impurity
(4,5)substitutional
impurities
panah arah stress lokal yang diakibatkan
oleh defects
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Impurity
Impurity atom pengotor yang ada di dalam material
Semua logam sebenarnya tidaklah murni. Very pure metals 99.9999%
- one pengotor (impurity) per 106 atoms
Adanya pengotor mungkin disengaja atau tidak
Carbon in small amounts in iron makes steel. It is stronger.
Boron in silicon change its electrical properties.
Alloys paduan dua logam atau lebih
Sterling silver dibuat dari 92.5% silver 7.5% copper alloy. Sifatnya lebih kuat daripada perak
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Bagaimana Ada Impurity dalam logam?
Solid solutions
Host (Solvent) melarutkan komponen dalam jumlah kecil (Solute).
Kemampuan untuk melarutkan disebut Solubility.
Solvent: komponen terbesar dalam campuran
Solute: komponen dalam jumlah kecil di campuran
Solid Solution: homogeneous
maintain crystal structure
randomly dispersed impurities
(substitutional or interstitial)
Second Phase
solute atoms added: new compounds or structures form or solute forms local precipitates
Sifat larutan padat dipengaruhi oleh impurities, concentration, temperature and pressure
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Larutan Padat Subsitusi (Substitutional
Solid Solutions)
Factors yang mempengaruhi kelarutan
atom:
(Solubility limit maximum dapat larut)
Atomic size: need to fit solute and solvent atomic radii should be within ~ 15%
Crystal structure: solute and solvent the same
Electronegativities: should be comparable (otherwise new inter-metallic phases
favored)
Valency: If solute has higher valency than solvent, generally more goes into solution
Ni
Cu
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Larutan Padat Interstisi (Interstitial Solid
Solutions)
Factors for high solubility:
FCC, BCC, HCP: void space between host (matrix) atoms relatively small
atomic radius of solute should be
significantly less than solvent
Max. concentration 10%, (2% for C-Fe)
Carbon
interstitial
atom in BCC
iron
Interstitial solid solution of C in BCC Fe ( phase).
C small enough to fit (some strain in BCC lattice).
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Composition / Concentration
atom percent (at %): useful in understanding material at atomic level
Number of moles (atoms) of one element relative to
total number of moles (atoms) in alloy.
2 component: concentration of element 1 in at. %:
Weight Percent (wt %)
Weight of one element relative to total alloy weight
2 components: concentration of element 1 in wt. %
100mm
mC
21
11
100nn
nC
21
1
mm
m'
1
nm1= number density = m1/A1 (m1 = weight in
grams of 1, A1 is atomic weight of element 1)
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Dislocations = Linear Defects
Interatomic bonds significantly distorted in
immediate vicinity of dislocation line
(Creates small elastic deformations of lattice at
large distances.)
Dislocations affect mechanical properties
Discovery in 1934 by Taylor, Orowan and Polyani
marked beginning of our understanding of
mechanical properties of materials
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Interfacial Defects External Surfaces
Surface atoms unsatisfied bonds
higher energies than bulk atoms
Surface energy, (J/m2)
Surface areas try to minimize (e.g. liquid drop)
Solid surfaces can reconstruct to satisfy atomic
bonds at surfaces.
Grain Boundaries
Polycrystalline: many small crystals or grains.
Grains have different crystallographic orientation.
Mismatches where grains meet.
1. Surfaces and interfaces are reactive
2. Impurities tend to segregate there.
3. Extra energy associated with interfaces
larger grains tend to grow by diffusion of atoms at expense of smaller grains, minimizing energy.
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High and Low Angle Grain Boundaries
Misalignments of atomic planes between grains Distinguish low and high angle grain boundaries
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Bulk or Volume Defects
Pores: affect optical, thermal, mechanical properties
Cracks: affect mechanical properties Foreign inclusions: affect electrical,
mechanical, optical properties
Cluster of microcracks in a
melanin granule irradiated
by a short laser pulse.
Computer simulation by L.
V. Zhigilei and B. J.
Garrison.